1. Field of the Invention
The present invention relates to a safety device for an air balancing hoist, which can prevent a sudden rise of a hook of the hoist due to rapid expansion of compressed air when a load or workpiece is accidentally loosed or released from the hook while performing a lifting-up operation using the air balancing hoist for lifting or lowering the load or workpiece through forward or reverse rotation of a reel drum on which a wire is wound by means of supply of the compressed air.
More specifically, the present invention relates to a safety device for an air balancing hoist, wherein upon sudden rotation of a reel drum due to accidental release of a load or workpiece from a hook during a lifting-up operation, a braking pawl attached to the reel drum is moved in an outward radial direction due to centrifugal force and then locked into a ratchet wheel fixed to an end cap, thereby braking the reel drum.
2. Description of the Prior Art
As schematically shown in FIG. 1, a conventional air balancing hoist includes a cylindrical housing 2 with a chamber 1, which compressed air is introduced into or discharged from, formed therein; a ball screw 5 centrally positioned in the housing 2 and with both ends thereof secured by fastening members 3, 4 so as not to rotate; and an end cap 7 mounted on one end of the housing 2 to hermetically seal the chamber 1 and provided with a manifold 6 attached to an outer side of the end cap. The manifold 6 has holes through which the compressed air is introduced and discharged.
The air balancing hoist further includes a piston 8 reciprocating along an inner periphery of the housing 2 by means of the compressed air introduced into and discharged from the chamber 1 through the manifold 6, and a reel drum 12 with one end thereof secured on a ball nut 9 engaged with the ball screw 5 to be movable along the ball screw and with the other end thereof rotatably supported by the piston 8 through a thrust bearing 10. Upon operation of the piston 8, the reel drum 12 rotates while moving along the ball screw 5 and thus winds or unwinds the wire 11.
Therefore, when a user presses an xe2x80x9cupxe2x80x9d lever on a push-button switch, the compressed air is discharged from a source such as a compressor and is introduced into the chamber 1 of the housing 2 through the manifold 6. Accordingly, the piston 8 moves along the inner periphery of the housing 2 within a predetermined range of stroke in the left direction in FIG. 1.
The linear movement of the piston 8 causes the reel drum 12 supported by the piston 8 through the thrust bearing 10 and supported by the ball screw 5 through the ball nut 9 to be axially moved and simultaneously to be rotated. Thus, the reel drum 12 winds up the wire 11 on the outer periphery thereof.
Thus, the load or workpiece hooked on a hook assembly secured at an end of the wire 11 can be lifted.
On the contrary, upon manipulation of a xe2x80x9cdownxe2x80x9d lever on the push button switch, the compressed air within the chamber 1 is discharged through the manifold 6 to the atmosphere. Then, the reel drum 12 is rotated while moving along the ball screw 5 in the right direction in FIG. 1, and thus, the wire 11 which has been wound on the reel drum 12 is unwound.
Accordingly, the workpiece hooked on the hook assembly can be lowered.
However, in the aforementioned air balancing hoist, if the workpiece hooked on the hook assembly is accidentally released during the lifting-up operation, the compressed air within the chamber 1 is rapidly expanded and then the reel drum 12 is rotated fast. Thus, the hook assembly is rapidly raised. Consequently, there is a problem in that upon the rapid raise of the hook assembly, the hook or a jig secured thereon may strike an operator, resulting in a safety accident.
There has been proposed some apparatuses for solving the problems. U.S. Pat. No. 5,522,581 (application Ser. No. 182,785) assigned to Zimmerman International Corporation discloses a cam-type braking mechanism for an air balancing hoist. The braking mechanism is constructed such that a cam of a braking pawl, which is mounted to a pin structure integrally secured on an end cap and can be moved in an outward radial direction due to centrifugal force upon fast rotation of the reel drum, comes into friction contact with an inner surface of the end cap to brake and finally stop the reel drum.
However, in the cam-type braking mechanism of the ""581 patent, a smaller contact area for braking the reel drum is generated when the cam of the braking pawl comes into friction contact with the inner surface of the end cap upon fast rotation of the reel drum during a lifting-up operation. This results in low braking force. Thus, the fast rotating reel drum cannot be instantaneously braked, but can be completely stopped after the processes of friction contact with slippage therebetween have been repeated three or four times. Accordingly, there is a disadvantage of deterioration in safety thereof.
On the other hand, U.S. Pat. No. 5,553,832 (application Ser. No. 284,800) assigned to Knight Industries Incorporation discloses a safety device for an air balancing hoist. That is, there is provided a braking apparatus wherein a braking pawl is fixed to a side of a reel drum in such a manner that the pawl can be moved in the outward radial direction due to centrifugal force, and an inner periphery of a cylindrical sleeve surrounding the reel drum is formed with longitudinal scalloped recesses corresponding to the braking pawl so that the braking pawl can be releasably locked into the recesses due to the centrifugal force upon fast rotation of the reel drum, thereby braking the rotation of the reel drum.
However, in the safety device of the ""832 patent, the recesses are formed in the inner periphery of the sleeve at a predetermined interval. Accordingly, when the braking apparatus is activated due to the fast rotation of the reel drum during the lifting-up operation and brakes the rotation of the reel drum, the reel drum cannot be instantaneously braked at portions where the recesses are not formed, and thus, continues to rotate fast. After the braking pawl comes into friction contact with the recesses only at portions where the recesses are formed, the braking pawl can brake and finally stop the reel drum. Consequently, there is also a disadvantage of deterioration in safety thereof.
Therefore, the present invention is conceived to solve the aforementioned problems in the prior art. An object of the present invention is to provide a safety device for an air balancing hoist, wherein upon fast rotation of a reel drum during a lifting-up operation, a braking pawl is locked into a braking wheel fixed to an end cap due to centrifugal force and then brakes the reel drum, thereby enhancing braking force and instantaneously braking the reel drum by means of prevention of slippage and disengagement of the pawl.
Another object of the present invention is to provide a safety device for an air balancing hoist, wherein upon fast rotation of a reel drum during a lifting-up operation, the reel drum can be instantaneously braked even at any positions within a range of lifting stroke, thereby ensuring safety to protect an operator.
A further object of the present invention is to provide a safety device for an air balancing hoist, wherein it is not necessary to machine a braking wheel and a braking pawl, thereby simplifying their configurations, reducing the number of processes, and thus, saving on the production costs.
A still further object of the present invention is to provide a safety device for an air balancing hoist, wherein it is not necessary to disassemble and reassemble the air balancing hoist in order to set a braking apparatus to an original state after braking the reel drum, and interior components such as a housing are prevented from being broken or damaged upon braking of the reel drum, thereby ensuring semipermanent use thereof.
In order to achieve the objects of the present invention, there is provided a safety device for an air balancing hoist including a housing with a chamber, which compressed air is introduced into and discharged from according to manipulation of a switch, formed therein and with end caps mounted on both ends of the housing, a piston reciprocating along an inner periphery of the housing upon supply and discharge of the compressed air to and from the chamber, a ball screw installed in the housing so that it penetrates the piston and is secured to the end caps so as not to rotate, and a reel drum with one end thereof rotatably supported by the piston through a bearing and with the other end thereof secured on a ball nut rotatably engaged with the ball screw so that the reel drum is rotated to wind up a wire when moving along the ball screw, comprising a braking wheel secured on an inner surface of the end cap and with a ratchet longitudinally formed on an outer surface of the wheel over the length corresponding to a range of movable distance of the reel drum; and a braking pawl radially pivotably mounted on a side of the reel drum to be releasably locked into the ratchet for braking the reel drum upon fast rotation of the reel drum during a lifting-up operation.
According to a preferred aspect of the present invention, the safety device for the air balancing hoist further comprises a fixing pin provided in the reel drum for preventing the braking pawl from being pivoted in a reverse direction.
According to another preferred aspect of the present invention, the safety device for the air balancing hoist further comprises an elastic member for resiliently biasing the braking pawl to an original state in which the pawl remains in non-contact with the ratchet so that the pawl can be disengaged from the ratchet after completing the braking of the reel drum by means of the releasable locking with the ratchet when braking the reel drum.